A number of missiles and missile-related hardware items use transparent dome or window components. For example, the missile uses a dome component which acts as a port for radiation sensing instruments such as infrared sensors or ultraviolet sensors. The missile is commonly launched from a launch tube which includes a sealing window intended to rupture upon launch but which should be transparent to infrared or ultraviolet radiation. Materials used in the past for such domes and windows, particularly MgF.sub.2, have a degree of transmittance in the infrared region, but are somewhat deficient in transmittance in ultraviolet wavelengths. Additionally, MgF.sub.2 is, for some purposes, somewhat deficient in certain mechanical properties. In this regard, MgF.sub.2 domes and launch windows have a knoop hardness of about 576 kg mm.sup.-2 and a flexural strength of about 12.5 kpsi.
A material for a launch tube window or a missile dome preferably is also relatively stable under conditions of high temperature, corrosive environments, such as oxidizing environments, or acidic environments, is substantially insoluble in water, can be polished to a high degree of polish, is stable under prolonged exposure to ultraviolet light, and is resistant to abrasion or erosion, particularly when subjected to dust, sand, or water droplets at a velocity of about 500 meters per second
Launch tube windows have an additional constraint in that the launch tube window should have sufficient strength and hardness to retain its integrity and transparency during ordinary storage and transport conditions, but which will rupture at a desired overpressure level, so that the window will disintegrate during launch of the missile.
Materials which have been used for producing a transparent body include metal fluorides, particularly magnesium fluoride (U.S. Pat. No. 3,589,880 issued June 29, 1971 to Clark; U.S. Pat. No. 3,294,878 issued Dec. 27, 1966 to Carnall, Jr., et al.; U.S. Pat. No. 3,431,326 issued Mar. 4, 1969 to Letter), aluminum oxynitride (U.S. Pat. No. 4,520,116 issued May 28, 1985 to Gentilman, et al.), aluminum niobate or tantalate (U.S. Pat. No. 4,047,960 issued Sept. 13, 1977 to Reade), and solid solutions of alumina, silica and other oxides (U.S. Pat. No. 4,009,042 issued Feb. 22, 1977 to Wittler) and alumina, with minor amounts of spinel (U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 to Coble).
Previous methods of ceramic preparation which have included a hot-press or closed-porosity step followed by a hot isostatic press step have included U.S. Pat. No. 4,461,750 issued July 24, 1984 to Chess, et al., U.S. Pat. No. 4,524,138 issued June 18, 1985 to Schwetz, et al., and U.S. Pat. No. 3,853,973 issued Dec. 10, 1974 to Hardtl, et al.
Methods have also been developed for production of transparent bodies substantially from a magnesia-alumina spinel. U.S. Pat. No. 3,974,249 issued Aug. 10, 1976 to Roy, et al., U.S. Pat. No. 768,990 issued Oct. 30, 1973 to Sellers, et al., U.S. Pat. No. 3,531,308 issued Sept. 29, 1970 to Bagley. Polycrystalline bodies of spinel are, in general, more easily formed than single-crystal or fusion-cast spinel or sapphire.
Previous materials and methods for production of a sintered transparent body have suffered from a number of difficulties. These materials have been deficient in transmittance in certain wavelength ranges, particularly ultraviolet ranges, for example, wavelengths from about 0.2 micrometers (microns) to about 0.4 microns, as well as visible and infrared wavelengths up to about 6 microns.
Previous materials were susceptible to abrasion or erosion, for example, from high velocity impaction of dust or sand particles or rain or cloud droplets.
Previous materials were often unstable under conditions of long exposure to ultraviolet light, such that exposure to sunlight or to ultraviolet light with an intensity of about 700 microwatts/cm.sup.2, on the order of 0.25 hours or more caused a reduction of the transmittance properties of the material.
Previous materials have been difficult to form with the desired structural strength. In some applications it is desired to produce a transparent window which can withstand mechanical stress on the order of a pressure of about 15 psi (0.1 MPa), but which will preferably rupture when subjected to a pressure of about 25 psi (0.17 MPa) or more.
Certain previous materials, e.g. MgO, are hygroscopic and become cloudy upon exposure to moisture, rendering the optical qualities of the material unacceptable.
Previous production methods have been costly to practice and have required a number of difficult steps making the windows impractical to produce in quantity.
Accordingly, there is a need for material which displays good ultraviolet transmittance, possesses elevated strength and hardness characteristics, and is resistant to erosion, thermal or chemical degradation, and the like.